Due to the nature of the two-stroke cycle, a load reversal on a journal bearing of a two-stroke engine such as a wristpin may never occur during the normal speed and load range operation of the engine, or the duration of a load reversal might be relatively short. In these circumstances, it is difficult to replenish the bearings with lubricating oil (“oil”). Furthermore, given limited angular oscillation of the bearing, oil introduced between the bearing surfaces does not completely fill the bearing. Eventually the bearing begins to operate in a boundary layer lubrication mode (also called “boundary lubrication mode”), which leads to excess friction, wear, and then bearing failure.
A representative two-stroke cycle engine is embodied in the opposed-piston engine 8 of FIG. 1. The engine 8 includes one or more cylinders such as the cylinder 10. The cylinder 10 is constituted of a liner (sometimes called a “sleeve”) retained in a cylinder tunnel formed in a cylinder block. The liner includes a bore 12 and longitudinally displaced intake and exhaust ports 14 and 16, machined or formed in the liner near respective ends thereof. Each of the intake and exhaust ports includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid portion of the cylinder wall (also called a “bridge”).
One or more injection nozzles 17 are secured in threaded holes that open through the sidewall of the liner, between the intake and exhaust ports. Two pistons 20, 22 are disposed in the bore 12 of the cylinder liner with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred to as the “intake” piston because of its proximity to, and control of, the intake port 14. Similarly, the piston 22 is referred to as the “exhaust” piston because of its proximity to, and control of, the exhaust port 16. The engine includes two rotatable crankshafts 30 and 32 that are disposed in a generally parallel relationship and positioned outside of respective intake and exhaust ends of the cylinder. The intake piston 20 is coupled to the crankshaft 30 (referred to as the “intake crankshaft”), which is disposed along an intake end of the engine 8 where cylinder intake ports are positioned; and, the exhaust piston 22 is coupled to the crankshaft 32 (referred to as the “exhaust crankshaft”), which is disposed along an exhaust end of the engine 8 where cylinder exhaust ports are positioned.
Operation of a two-stroke cycle, opposed-piston engine with one or more cylinders is well understood. Using the engine 8 as an example, each of the pistons 20, 22 reciprocates in the bore 12 between a bottom center (BC) position near a respective end of the liner 10 where the piston is at its outermost position with respect to the cylinder, and a top center (TC) position where the piston is at its innermost position with respect to the cylinder. At BC, the piston's end surface 20e, 22e is positioned between a respective end of the cylinder, and its associated port, which opens the port for the passage of gas. As the piston moves away from BC, toward TC, the port is closed. During a compression stroke each piston moves into the bore 12, away from BC, toward its TC position. As the pistons approach their TC positions, air is compressed between their end surfaces. Fuel is injected into the compressed air. In response to the pressure and temperature of the compressed air, the fuel ignites and combustion follows, driving the pistons apart in a power stroke. During a power stroke, the opposed pistons move away from their respective TC positions. While moving from TC, the pistons keep their associated ports closed until they approach their respective BC positions. In some instances, the pistons may move in phase so that the intake and exhaust ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
In FIG. 1, the pistons 20 and 22 are connected to the crankshafts 30 and 32 by respective coupling mechanisms 40 including journal bearings 42. The journal bearings 42 are continuously subjected to non-reversing, compressive loads during operation of the engine 8. Related U.S. patent application Ser. No. 13/776,656 describes and illustrates a solution to the problem of non-reversing compressive loads for two-stroke cycle, opposed-piston engines. The solution includes a rocking journal bearing (also called a “rocking bearing” or a “biaxial bearing”), which is incorporated into the engine 8 of FIG. 1. Each journal bearing 42 of each coupling mechanism 40 of the engine 8 is constructed as a rocking journal bearing. Referring to FIGS. 1 and 2, a coupling mechanism 40 supports a piston 20 or 22 by means of a rocking journal bearing 42 including a bearing sleeve 46 having a bearing surface 47, and a wristpin 48. The wristpin 48 is retained on the small end 49 of a connecting rod 50 for rocking oscillation on the bearing surface of the sleeve by threaded fasteners 51 received in threaded holes 52. The large end 53 of the connecting rod 50 is secured to an associated crankpin 54 of a respective one of the crankshafts 30, 32 by conventional fasteners (not shown).
As seen in FIG. 3, the wristpin 48 is a cylindrical piece that comprises a plurality of axially-spaced, eccentrically-disposed journal segments. A first journal segment J1 comprises an annular bearing journal surface formed in an intermediate portion of the wristpin, between two journal segments J2. The two journal segments J2 comprise respective annular bearing journal surfaces formed on opposite ends of the wristpin, on respective sides of the journal segment J1. The journal segment J1 has a centerline A. The journal segments J2 share a centerline B that is offset from the centerline A of journal segment J1. As seen in FIG. 3, the sleeve 46 is a semi-cylindrically shaped piece with a bearing surface that includes a plurality of axially-spaced, eccentrically-disposed surface segments. A first surface segment J1′ comprises an arcuately-shaped bearing surface formed in an intermediate portion of the sleeve, between two surface segments J2′. The two surface segments J2′ comprise arcuately-shaped bearing surfaces formed at opposite ends of the sleeve, on respective sides of the surface segment J1′. The surface segment J1′ has a centerline A′. The wristpin 48 is mounted to the small end 49 of the connecting rod 50 and the sleeve is mounted to an internal structure of the piston (not shown), such that corresponding bearing segment sets J1-J1′ and J2-J2′ are in opposing contact. Thus disposed, the opposing corresponding segment sets J1-J1′ and J2-J2′ may also be called “bearing interfaces”.
In operation, as the piston to which they are mounted reciprocates between TC and BC positions, oscillatory rocking motion between the wristpin 48 and the sleeve 46 causes the bearing interfaces J1-J1′ and J2-J2′ to alternately receive the compressive load. The bearing surface segments receiving the load come together and the bearing surface segments being unloaded separate. Separation enables a film of oil to enter space between the separating bearing surfaces. The point at which the compressive load is shifted from one to the other set of bearing segments is referred to as a “load transfer point.” During one full cycle of the two-stroke cycle engine, this point is traversed twice by each piston, once when the piston moves from TC to BC (that is to say, during the power stroke), and again when the piston moves from BC to TC (during the compression stroke). For illustration and as an aid in visualization, but without limiting the following disclosure, the load transfer points of the pistons may occur at or near crankshaft positions of 0° (when the pistons pass through their respective TC locations) and 180° (when the pistons pass through their respective BC locations).
With reference to FIGS. 1 and 2, the rocking journal bearings are constructed to enable provisioning and distribution of oil at pressures adequate to lubricate the rocking bearing interfaces with a continuous oil film thick and widespread enough to support heavy loading, thereby enhancing the durability of the bearing. The construction of the wristpin 48 includes a gallery 60 which receives and distributes oil for lubricating the bearing interfaces (J1-J1′ and J2-J2′). The gallery 60 is fed pressurized oil from a pumped oil source. The wristpin 48 includes an oil inlet into, and multiple oil outlets from, the gallery 60. The gallery 60 receives the pressurized oil through an inlet opening 62 that opens through a portion of the wristpin surface that is out of contact with the sleeve surface segments. The pressurized oil is delivered via a high-pressure oil passage 64 in the connecting rod. Pressurized oil is provided to the bearing interfaces (J1-J1′ and J2-J2′) from the gallery 60 through outlets that act through a portion of the wristpin surface in contact with the sleeve's bearing surface during oscillation of the bearing. An influx of pressurized oil into the gallery 60 provides a continuous supply of pressurized oil to the bearing during operation of the engine.
As seen in FIG. 4, oil is circulated to the bearing interfaces via a network of oil grooves formed in the bearing surface 47 of the sleeve 46 for transporting oil to the bearing surface. The network includes circumferential oil grooves 70 for transporting oil in a circumferential direction of the bearing surface. The circumferential oil grooves 70 are formed in the bearing surface at the borders between the central surface segment J1′ and the lateral surface segments J2′. The network further includes circumferentially-spaced, axial oil grooves 72 and 73, each for transporting oil in an axial direction of the bearing surface. The axial oil grooves are formed in the bearing surface transversely to and intersecting with the circumferential oil grooves 70. Each of the axial oil grooves 72 and 73 runs across the central surface segment J1′ and extends at least partially into each of the lateral surface segments J2′. Chamfers 74 may be formed along opposing lateral peripheries of the bearing surface 47. FIGS. 5A-5C show a prior art rocking journal wristpin constructed to deliver oil to the sleeve's bearing surface 47.
FIGS. 5A-5C show a rocking journal wristpin 80 with an outer surface 82 having journal segments J1 and J2 that contact surface segments J1′ and J2′ of the sleeve bearing surface 47 during oscillation of the bearing. The journal segments J1 and J2 are separated by circumferential grooves 85 in the wristpin outer surface 82. Outlet passages formed in the wristpin provide pressurized oil to the sets of surface segments during relative oscillatory motion between the sleeve and wristpin. First oil outlet passages 86 for delivering pressurized oil are formed in the contacting portion of the outer surface 82 and extend through the sidewall of the wristpin in the circumferential grooves 85 in a radial direction of the journal segment J1 and open into an oil gallery 88. An oil inlet 90 to the oil gallery 88 and the first oil outlet passages 86 are axially spaced, in diametrical opposition. Second oil outlet passages 92 are formed through the sidewall of the wristpin, outside of the circumferential grooves 85, and open into the oil gallery 88. The second oil outlet passages 92 are arranged in an axial array such that there is at least one second oil outlet passage located in each journal segment J1 and J2. The wristpin 80 is assembled to the sleeve 46 of FIG. 4 with the journal segments J1-J2 in opposition to the surface segments J1′-J2′ and the circumferential grooves 85 of the wristpin aligned with the circumferential grooves 70 of the sleeve. As per FIGS. 4 and 5A, during operation of the engine, the first oil outlet passages 86 continuously supply pressurized oil to the network comprising circumferential oil grooves 70, which flows to the axial oil grooves 72 and 73. As relative oscillation occurs between the wristpin 80 and the sleeve 46, pressurized oil flows to the space between the separated segments continuously from the oil grooves 70, 72, and 73 and intermittently from the second oil outlet passages 92 as the journal segments in which they are located separate from their opposing surface segments of the sleeve.
Thus, the prior art wristpin oil delivery construction provides a constant supply of pressurized oil to the oil grooves 70, 72 and 73 in the sleeve surface; and, the oil grooves continuously transport oil to the journal segments. However, a continuous supply of pressurized oil results in a high level of oil flow from the ends of the circumferential grooves 70. This excess oil is detrimental to the performance of the engine for at least two reasons. First, the continuous provision of pressurized oil requires pumping work to supply the oil to the grooves, which reduces the engine's efficiency. Second, the oil comes in contact with the rotating and reciprocating machinery while returning to an engine oil sump. Extra parasitic drag caused by oil returning to the sump and interacting with a swirling cloud of air in the crankcase of the engine created by the high-speed rotation of the crankshafts, (“windage”), results in frictional losses. At 3,000 RPM, for example, each crankshaft must rotate 50 times per second. As the crankpins and counterweights rotate at such high speeds, they create a swirling cloud of air around them. As a result windage friction losses occur when excess oil is caught up in this turbulent air, drawing energy from the engine to spin the oil mist. Windage may also inhibit the migration of oil into the sump and back to the oil pump, creating lubrication problems. It is therefore desirable to minimize the amount of excess pressurized oil that flows through the rocking bearing journals of an engine.